General purpose x-ray equipment typically has one or more movable arms upon which an x-ray source and x-ray detector may be mounted. The movable arms are designed to provide flexibility in positioning an x-ray beam passing between the x-ray source and detector, to a particular location in the patient and at a specific angle to the patient. The location and angle may be important in maximizing the diagnostic information contained in the image to be obtained.
Frequently, a series of different images of different regions and at different angles will be desired as part of a single routine or procedure. For example, with dual energy x-ray equipment used for measuring bone density, a common procedure may require the measurement of the density of the bones of the spine and hip at two different angles.
In some x-ray systems, positioning of the arms is done manually with the operator grasping handles near the radiation source, for example, to move the arm to its new position. The arm is typically counterbalanced, both so as to be easily movable and so as to remain at the new position without external support.
In more complex x-ray equipment, or x-ray equipment with larger radiation sources and detectors, the movement of the arms is done by motors that may be controlled either by a handheld pendant, held by the operator, or by instructions generated under computer control. The use of motors to move the arm permits the use of more sophisticated x-ray machine architectures, for example, architectures employing C-arms which improve patient access, but which are intrinsically unbalanced. The use of motors to control the motion of the arm also permits more accurate positioning of the arm, and is necessary for scanning systems where the source and detector must be precisely translated during the exposure.
When a motorized x-ray system is repositioned under operator control, the operator must observe all moving portions of the radiographic equipment to insure that there are no collisions between stationary and moving portions of the equipment. Likewise, the operator must ensure that no portion of the equipment strikes the patient. This is not always easy because the source and detector may be moving in opposite directions at spatially distant points. PG,4
This problem of properly monitoring the motion of the x-ray machine becomes increasingly difficult as the number of axes of motion permitted by the equipment increases. For this reason, pressure sensitive switches are typically placed at likely points of contact between components of the radiographic system. Such switches may have conductor pairs held in spaced opposition by an elastomer so as to contact each other only when the elastomer is deformed in a collision. An alternative switch is a sealed tube holding air whose pressure is monitored to detect deformation of the tube as might be caused by a collision.
When the radiographic system permits the movement of multiple axes, the best manner of moving the arm between two points is not always apparent and the operator's control of the arm to realize that motion may be slow and inefficient. This is particularly true where a given position may be apparently reached with more than one set of axes motions but where motion limits on one or more axes produces a "dead end" requiring a time consuming backtracking in order to reach the desired position.